INT32-C. Ensure that integer operations do not result in an overflow

Integer values used in any of the the following ways must be guaranteed correct:

• as an array index
• in any pointer arithmetic
• as a length or size of an object
• as the bound of an array (for example, a loop counter)
• in security critical code

Most integer operations can result in overflow if the resulting value cannot be represented by the underlying representation of the integer. The following table indicates which operators can result in overflow:

The following sections examine specific operations that are susceptible to integer overflow. The specific tests that are required to guarantee that the operation does not result in an integer overflow depend on the signedness of the integer types. When operating on small types (smaller than int), integer conversion rules apply. The usual arithmetic conversions may also be applied to (implicitly) convert operands to equivalent types before arithmetic operations are performed. Make sure you understand implicit conversion rules before trying to implement secure arithmetic operations.

Exceptions

Unsigned integers can be allowed to exhibit modulo behavior if and only if

1. the variable declaration is clearly commented as supporting modulo behavior
2. each operation on that integer is also clearly commented as supporting modulo behavior

If the integer exhibiting modulo behavior contributes to the value of an integer not marked as exhibiting modulo behavior, the resulting integer must obey this rule.

Risk Assessment

Integer overflow can lead to buffer overflows and the execution of arbitrary code by an attacker.

References

[[ISO/IEC 9899-1999] Section 6.5, "Expressions," and Section 7.10, "Sizes of integer types <limits.h>"
[[Seacord 05]] Chapter 5, "Integers"
[[Warren 02]] Chapter 2, "Basics"
[[Viega 05]] Section 5.2.7, "Integer overflow"
[[Dowd 06]] Chapter 6, "C Language Issues" (Arithmetic Boundary Conditions, pp. 211-223)